1. Field of the Invention
The present invention relates to a leading edge structure for an airfoil such as a main wing, a horizontal stabilizer, a vertical fin, an elevator or a rudder of an aircraft, and a method of fabricating such a leading edge structure. More particularly, the present invention relates to a leading edge structure for an aircraft, integrally formed with a hot air passage of an anti-icing system, and a method of fabricating the leading edge structure.
2. Description of the Related Art
The leading edge structure of an airfoil such as a main wing, a horizontal stabilizer, a vertical fin, an elevator or a rudder of an aircraft must have an anti-icing system to prevent the ice formation on the leading edge structure. The anti-icing system supplies hot air extracted from an engine compressor into the interior space of the leading edge structure covered by an outer skin so as to flow along the inner surface of the outer skin to raise the surface temperature of the outer skin and to prevent the ice formation on the outer surface of the leading edge structure.
Such a leading edge structure is disclosed in JP-A-9-71298. Referring to FIG. 7 showing the prior art leading edge structure 100 for an aircraft disclosed in JP-A-9-71298 in a partially cutaway perspective view, an inner skin 102 of a fiber-reinforced plastic and a front wall 103 are disposed on the inner side of an outer skin 101 of a fiber-reinforced plastic. The outer skin 101, the inner skin 102 and the front wall 103 are united by bonding to define a heating chamber 104 in the leading edge structure 100. A rear wall 105 is disposed behind the front wall 103 to define a hot air discharge chamber 106 between the front wall 103 and the rear wall 105. The heating chamber 104 is partitioned by the front wall 103 into a hot air jetting section 104a on the front side and a hot air passage section 104b on the rear side. An anti-icing duct 108 provided with a plurality of air jetting holes 108a is extended in the hot air jetting section 104a. Hot air flows through the anti-icing duct 108. Hot air ejected through the hot air jetting holes 108a of the anti-icing duct 108 flows from the hot air jetting section 104a into the hot air passage section 104b, flows along hot air passages 104c formed in the hot air passage section 140b into the hot air discharge chamber 106, and is discharged through a discharge hole formed in the tip of the wing into the atmosphere. The formation of ice on the leading edge structure 100 is prevented by thus making the hot air flow along the inner surface of the outer skin 101 of the leading edge structure 100.
A method of fabricating this prior art leading edge structure 100 will be described hereafter. As shown in FIG. 8A, an outer skin prepreg sheet 112 for forming the outer skin 101 is placed on a shaping surface of a leading edge skin mold 111 having a shape corresponding to the external shape of the outer skin 101. A comb-shaped silicone block 113 is placed as shown in FIG. 8B on the inner surface of the outer skin prepreg sheet 112 before hardening. An inner skin prepreg sheet 114 for forming the inner skin 102 is placed on the silicone block 113 so that parts thereof are forced into spaces between adjacent teeth to form straightening fins defining the hot air passages 104c. The leading edge skin mold 111 holding the outer skin prepreg sheet 112, the silicone block 113 and the inner skin prepreg sheet 114 is placed in a vacuum bag, and the vacuum bag is evacuated, the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114 are heated and compressed to unite the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114 by bonding the straightening fins of the inner skin prepreg sheet 114 to the outer skin prepreg sheet 112. Subsequently, the silicone block 113 is removed, the front wall 103 formed by a separate process is joined to the assembly of the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114 (FIG. 8C), and the anti-icing duct 108 is attached to the front wall 103 to complete the leading edge structure 100.
Aluminum alloys have been used as materials for forming the leading edge structures of aircrafts because each of aluminum alloys has a small specific gravity and a high strength. FIG. 9 is a perspective view of an aluminum alloy leading edge structure 120, FIG. 10 is a sectional view taken on line X--X in FIG. 9, and FIG. 11 is a sectional view taken on line X1--X1 in FIG. 9. The aluminum alloy leading edge structure 120 will be described with reference to FIGS. 9, 10 and 11. An inner skin 122 formed by bending a corrugated sheet is extended along the inner surface of an outer skin 121 to form a hot air passage 123 extending along the inner surface of the outer skin 121 structure. A hot air outlet opening 122a is formed in a part of the inner skin 122 corresponding to the front edge of the leading edge structure 120. A plurality of ribs 124 are attached to the inner skin 124 to divide a space defined by the inner skin 122 into a plurality of sections. A front wall 125 are attached to the rear edges of the inner skin 124. Each rib 124 is provided with an opening 124a. An anti-icing duct 127 is extended through the openings 124a of the ribs 124, and the anti-icing duct 127 is fastened to the ribs 124 by clips 128 formed by bending a sheet. As shown in FIG. 12 in a perspective view, the anti-icing duct 127 is provided with a plurality of air outlet openings 127a.
Hot air of a high temperature supplied into the anti-icing duct 127 is jetted through the air outlet openings 127a into the space defined by the inner skin 122 and the front wall 125. Then, the hot bleed air flows through the hot air outlet opening 122a formed in the inner skin 122 into the space between the outer skin 121 and the inner skin 122, flows through the same space along the inner surface of the outer skin 121 and is discharged through a discharge hole formed in the tip of the wing into the atmosphere. The hot air that flows along the inner surface of the outer skin structure 121 raises the surface temperature of the outer skin 121 to prevent the formation of ice on the surface of the leading edge structure 120.
The outer skin 121, the inner skin 122 and the rib 124 of the leading edge structure 120 are built by forming aluminum alloy sheets by press forming or roll forming, and finished by shaping the external shape by trimming. The clips 128 for fastening the anti-icing duct 127 to the ribs 124 are formed by bending an aluminum alloy sheet. The anti-icing duct 127 is formed by bending a tube of a titanium alloy or a stainless steel, and forming the plurality of air outlet openings 127a in the tube. The leading edge structure 120 is constructed by positioning these components of the leading edge structure 120 relative to each other by using jigs and adjusting shims, forming holes for receiving fasteners in the components, and fastening together the components with fastening means, such as rivets and clips.
When fabricating the former prior art leading edge structure 100, the leading edge structure is formed by bonding together and hardening the outer skin 101 of a fiber-reinforced plastic, the inner skin 102 of a fiber-reinforced plastic, and the front wall 103. Therefore, firm joints are formed between the outer skin 101 and the inner skin 102 by its own adhesive strength of the outer skin 101 and the inner skin 102. However, since only the front wall 103 and the rear wall 105 spaced from the front wall 103 are extended between the upper and the lower section of the outer skin of the leading edge structure, there is the possibility that the leading edge structure 100 cannot be formed in a required strength. Furthermore, fabrication of the leading edge structure 100 needs many processes and troublesome work because the leading edge structure 100 is fabricated by placing the outer skin prepreg sheet 112 for forming the outer skin 101 on the shaping surface of the leading edge skin mold 111 having a shape corresponding to the external shape of the outer skin 101, placing the comb-shaped silicone block 113 on the inner surface of the outer skin prepreg sheet 112, placing the inner skin prepreg sheet 114 for forming the inner skin 102 on the silicone block 113, placing the leading edge skin tool mold 111 holding the outer skin prepreg sheet 112, the silicone block 113 and the inner skin prepreg sheet 114 in the vacuum bag, evacuating the vacuum bag, heating and compressing the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114 to unite and harden the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114, removing the silicone block 113, joining the front wall 103 formed by a separate process to the assembly of the outer skin prepreg sheet 112 and the inner skin prepreg sheet 114, and attaching the anti-icing duct 108 to the front wall 103.
The outer skin 121, the inner skin structure, the plurality of ribs 124, the anti-icing duct 127 and the plurality of clips 128 respectively having different shapes need to be assembled when fabricating the latter prior art leading edge structure 120. Those components need to be formed in high accuracy and need expensive jigs to position the same correctly relative to each other, and the leading edge structure must be formed in an accurate external shape meeting severe aerodynamic requirements. The relative positions of the components must be adjusted by using shims, and much labor is required for forming the holes in the components and fastening together the components with fasteners, such as rivets, inserted in the holes. Furthermore, the dependence of strength only on load transfer through the joints is undesirable fin respect of the efficiency of load transfer between the components, fastening the anti-icing duct to the ribs 124 by the clips 127 in an narrow space in the leading edge structure requires difficult work and a great deal of skill, and increases assembling costs.
Since the anti-icing duct 127 must be heat-resistant, the same is made of a titanium alloy or a stainless steel. The titanium alloy of the stainless steel are hard to work and hence the formation of the air outlet openings 127a in the anti-icing duct 127 needs the frequent changes of boring tools, troublesome work and much labor.